1. Field of the Invention
The invention relates to a method and an apparatus for the injection of alcohol fuels (ethanol, methanol), more particularly for direct injection diesel engines, so that the alcohol fuels having poor spontaneous ignition qualities may be processed in engines without spark ignition.
2. Description of the Prior Art
When alternative alcohol fuels are used in direct injection diesel engines, it is necessary to raise the temperature in the combustion chamber of a direct injection diesel engine with a fuel having spontaneous ignition qualities (diesel fuel) during the injection phase of the alcohol fuel having poor spontaneous ignition qualities to such an extent that the spontaneous ignition temperature of the latter is reached.
It is known that special measures must be taken for the purpose of processing alcohol fuels having poor spontaneous ignition qualities in engines without spark ignition. This can be achieved, on the one hand, by lowering the temperature necessary for spontaneous ignition by admixing an ignition accelerator, such as, for example, kerobrisol or amyl nitrate. On the other hand, this can be achieved by measures taken in respect of the engine, that is, by raising the temperature level towards the end of the compression cycle to a level above the temperature of spontaneous ignition.
As follows from the polytropic equation for the temperature increase during compression, the temperature of the charge at the end of the compression cycle is essentially dependent upon the temperature at the time when the inlet valve is closed and upon the height of the compression ratio. In addition to raising the compression ratio, it is therefore important to effect an increase in the temperature of the intake air. The latter can either take place by heating the intake air or by mixing the same with hot exhaust gases which can either be taken from a separate burner or simply from the exhaust gas line of the engine.
In most diesel engines a comparatively large quantity of ignitable mixture will already be present on account of the long ignition lag at the time of ignition and this will result in combustion in sudden bursts having all the negative effects upon the driving apparatus and the noise radiation. The M process of MAN avoids this problem with wall addition of the fuel, guaranteeing that as soon as combustion starts only small quantities of fuel are processed and that further combustion proceeds in a controlled manner with the aid of an air vortex. At any rate it has hereby been shown that the start and warming up in the pure alcohol operation could only be achieved with the aid of a starting aid based on a starting pilot plant, for example. If, before the engine is switched off, in each case there is a switch over to diesel fuel, the later start can be realized in the pure diesel fuel operation. The injection equipment differs from that of the conventional multifuel engine because of the pump element which is enlarged on account of the lower thermal value.
The suction pipe injection of methanol, the diesel fuel-methanol-emulsion method and the direct injection of methanol have been investigated as further methods of forming a mixture for the purpose of operating direct injection diesel engines with alcohol fuels. In the case of suction pipe injection methanol is admixed with the air taken in in the suction pipe. The diesel fuel injected in a conventional manner takes over the ignition. As a result of various problems--for example, spark failures may result in the partial charge range of this method, so that the methanol constituent in this operating range has to be greatly reduced--it is possible, but only to a limited extent, to substitute diesel fuel for methanol by using suction pipe injection according to the present level of development. Similar problems also arose in the case of the emulsion method in which a mixture of diesel fuel and methanol was used instead of diesel fuel. As still to this day no method for preparing a stable mixture of diesel fuel and methanol is known, an emulsion must be produced in the fuel system by constant, intensive, mechanical mixing. As a result of the high level of vaporization heat the emulsified methanol has a detrimental effect upon the ignition lag of the diesel fuel and in the partial charge range increased HC and CO emissions result and even spark failures can occur.